The present embodiments relate to a magnetic resonance tomography system (MRT system) having at least one receive apparatus for receiving an electromagnetic and/or magnetic MR-HF signal that is radiated by a body to be examined.
An MRT system and a receive apparatus are known, for example, from U.S. Pat. No. 7,508,213 B2.
With a system for generating magnetic resonance tomograms, a body to be examined (e.g., the body of a person or animal or a material sample) is introduced into a magnetic field that is generated, for example, by a superconducting magnetic coil of the MRT system. The body is made to transmit a high frequency magnetic or electromagnetic response signal by emitting a high frequency magnetic alternating field. The frequency of the response signal is dependent on the field strength of the magnetic field and amounts approximately to 42 megahertz per Tesla. To distinguish between individual regions in the body, gradient fields cause individual voxels (e.g., volume elements) of the body to emit response signals that differ from one another by a few hundred kilohertz. The high frequency signal mixture emitted by the voxels is referred to, for example, as MR-HF signal.
The MR-HF signal is received by at least one receive coil element of the MRT system and is converted by the element into an electrical MR-HF signal. The entire overlaid signals of the individual voxel are to be demodulated from the high frequency range (HF range) to an intermediate frequency range or a basic frequency range. The electrical MR-HF signal of the receive coil element is amplified if necessary and fed to a mixer after an impedance adjustment, the mixer mixing down or demodulating the HF-MR signal into the intermediate frequency range or the base band. The signal demodulated into the intermediate frequency or basic frequency range is referred to below as an MR signal.
The receive coil elements are disposed in a receive apparatus that is located in the magnetic field of the MRT system. This makes it difficult to operate further complex electrical circuits in the receive apparatus. The relatively strong magnetic alternating fields render complex shielding measures necessary. Furthermore, the waste heat of such circuits causes a thermal signal noise that may disturb the MR-HF signal. Voluminous electrical circuits also render the receive apparatus unwieldy, which, particularly with devices for examining living patients, render it difficult for these receive apparatuses to be fixed to the patient for a long time. Furthermore, it may be unpleasantly hot for the patient on account of the waste heat of the devices.
The receive apparatus may thus solely be equipped with required components. Further components are accommodated in a magnetically shielded area (e.g., a control room of the MRT system). The receive apparatus may be connected to this evaluation device by way of coaxial cables. The coaxial cables guide the amplified electrical MR-HF signals out of the magnetic field. The amplified electrical MR-HF signals are then demodulated and further processed in the evaluation device. Since an MRT system may include a plurality of receive coil elements, the cable loom including coaxial cables is very unwieldy. On account of the plurality of transmitted signals, cross-talk between the individual coaxial cables may only be prevented with significant effort. A common mode current is to be prevented from being induced into the casing of the coaxial cable on account of the magnetic alternating fields of the magnet system. Common mode chokes or sheath current filters are thus used along the coaxial cable. On account of the large magnetic field strengths, these filters may not be provided by ferrite cores, but instead only in the form of resonant circuits (e.g., bazookas). In this way, the sheath current filters are to be at a distance from a quarter wavelength of the sheath currents. With a field strength of 3 Tesla, the distance therefore amounts to 15 cm, for example. This again renders the receive apparatus undesirably unwieldy.
An MRT system is described in U.S. Pat. No. 7,508,213 B2, in which an MR-HF signal is optically transmitted via optical waveguides out of the magnetic field into a control room of the MRT system. With this system, the receive apparatus includes an optical modulator, to the electrical control input of which a receive coil element is connected. A laser light with a constant light intensity is fed to the modulator. The light intensity of the laser light is modulated by the optical modulator as a function of the MR-HF signal generated by the receive coil element. The output signal of the optical modulator is transmitted via the optical waveguides to an opto electronic converter of a magnetically shielded evaluation device, which converts the optical signal back into an electrical MR-HF signal. The electrical MR-HF signal is then fed to a mixer for demodulation purposes.